High pressure fuel pump units typically comprise a pump head having an internal blind bore within which is arranged a piston reciprocally displacing. The pump head is provided with a turret member extending outwardly from the head and surrounding the bore. A coil spring compressed between a cam follower and the pump head is axially maintained on the turret. As the high pressure fuel leaks between the piston and the bore, even if the clearance is of microns, on oil lubricated pumps to prevent any mixing of fuel with oil into the cambox, the pump head is further provided with a return fuel gallery comprising an annular groove and a return channel extending from said groove.
A first problem is that the guiding bore is lengthy extending in the pump head and through the turret and, the very precise machining of the bore is difficult. Currently a typical clearance utilized ranges from 4.5 to 6.5 μm. A further problem is that the annular groove also requires difficult and costly machining.